Tropical Flowers and Tropical Forests: What Climate Change Means for Both

Scientists have been monitoring flowering activity at a tropical forest in Panama since 1982. When they first began weekly censuses of different plant species’ flowering patterns, they were not thinking about climate change. They just wanted to better understand the reproductive activity of tropical forest species.

The forest canopy at Barro Colorado Island, Panama during the dry season in February. A recent paper by Stephanie Pau and colleagues has shown that increasing flowering activity in Panama is linked to rising atmospheric carbon dioxide.

Fast forward to 2013 when I had the opportunity to study this multi-decadal record of flowering. I coupled the field-based flowering records of hundreds of species with satellite observations of cloud cover to examine the effect of light limitation on tropical forest flowering. Because tropical forests are generally warm and wet, some researchers have hypothesized that these forests are most limited by light, which plants need to photosynthesize. Studies in other tropical forests showed a “greening-up” using satellite images during droughts and suggested that this was due to reduced cloud cover and more light for photosynthesis. During photosynthesis, plants convert atmospheric carbon dioxide (CO2) to energy. They then allocate that energy to different components such as woody growth, as well as leaf, flower and seed production. Were these tropical species allocating this carbon to flowering during periods of more light availability?

I found that periods of reduced clouds and increased light were associated with more flowers, yet pronounced changes in cloudiness only occurred seasonally. But what was remarkable about this flowering record was it showed a steady increase in flowering over the past several decades.

A litter basket from the forest of Barro Colorado Island, Panama. A recent paper by Stephanie Pau and colleagues has shown that increasing flowering activity in Panama is linked to rising atmospheric carbon dioxide.

To understand this long-term trend, my collaborators and I followed up our initial study by comparing the relative effects of rainfall, light, temperature, atmospheric CO2, and El Niño activity on flowering over the past several decades. This study found that atmospheric CO2 clearly had the largest impact on the long-term increase in flowering compared to other climatic changes. However short-term peaks in flowering were associated with El Niños years because El Niños result in warm sunny conditions in Panama.

The effect of CO2 on forests should not be surprising given how important photosynthesis is to plants. But it is easy to forget that the entire Earth is experiencing a drastic chemical change in our atmosphere. We don’t see it or feel it like we do for heat waves or droughts.

Does this mean that climate change is good for tropical forests? The answer is no. First of all, we need to understand what more flowers mean for the entire life cycle of plants – seed production, dispersal, recruitment, growth, and survival. More flowers by themselves is not enough to indicate the overall integrity of the forest. Second, we need to understand species-specific responses to climate change. If some species are favored more than others, there may be shifts in the species compositions of these forests. Those shifts mean that the countless ecosystem functions that tropical forests perform, such as providing habitat and food for hundreds of species as well as carbon storage and biogeochemical cycling, will be altered in ways we can’t predict with high certainty.

Back in 1982 most scientists were not thinking about climate change. But now overwhelming data has accumulated, based on empirical observations, models, and theory, showing that increasing anthropogenic greenhouse gases in our atmosphere are altering ecosystems upon which we depend. These gases are invisible to us, but are radically changing the world we live in.

Dr. Stephanie Pau is an assistant professor in the Department of Geography. She studies biogeography, biodiversity conservation, climate change and remote sensing.